Monday, January 19, 2009

Sweetness and Light Dopamine

Just the taste of a sweet beverage can result in liking, wanting and sipping of more of the sweet beverage. (Please see the previous blogpost for details.) The liking-wanting-sipping phenomenon happens even when the sweet beverage is not actually consumed, and happens in less than a minute. What causes this reaction to the taste of sweet?

In 1989, L.H. Schneider observed that dopamine receptors in the brain are stimulated when rats are allowed to feed themselves sweet solutions. Since that time, many investigators have noticed a relationship between either the taste of sweet or the actual consumption of sweet and the response of dopamine receptors in the brain, both in rats and in humans.

Dopamine is a neurotransmitter found in the midbrain. It has many functions, but one of them is the ability to produce prolonged feelings of pleasure. Increased dopamine signaling is involved in the mechanism of addiction to cocaine, amphetamines, and nicotine. At a more moderate level, novel foods, sweet foods, and overeating also cause increases in brain dopamine.

In order to investigate possible rebound effects of overstimulation with dopamine followed by abrupt dopamine withdrawal, a group of rats was treated for five days with l-dopa (a substance which is converted to dopamine in the brain). The research is described in an article in the December 2008 issue of Nutrition & Metabolism. After treatment was completed, the rats in the previously-treated l-dopa group were compared with an untreated control group. Over the next 12 weeks both groups of rats were allowed to eat as much food as they desired. At the end of that time, the previously-treated rats had gained 15% more weight than the control group.

Why did this happen? The authors hypothesize that treatment with the dopamine precursor l-dopa caused overstimulation of the dopamine signaling system in the rats. This, in turn, caused downregulation of dopamine receptors and decreased endogenous dopamine production. When the l-dopa treatment ceased, the rats were left with few dopamine receptors and low endogenous dopamine production. To compensate for this, the rats used the mechanism of overeating to compensate for their relative dopaminergic deficiency.

Rats are not humans. Nevertheless, it is possible to suggest that eating or tasting sweet food causes an overstimulation of the dopamine signaling system. This produces a downregulation of dopamine signaling such that, in the absence of sweet, there is a noticeable decrease in energy, motivation and mood. Sweets are legal, cheap and easy to obtain. If the sweet-eater wishes to experience the pleasant feelings associated with dopamine overstimulation, it will be very easy to continue eating sweets. And if he is unable to wait several days to a week to allow the body to return to its normal level of dopamine production and receptor activity, it will be harder than he might have expected to eliminate sweet tastes from his diet.


Anonymous said...

GREAT POST. This explains what I have been feeling for sure. i have been quite anxious and have been really craving carbs. I mentioned on my blog it was probably due to some type of carb addiction and the buzz you get from carbs. That explanation shows me why.

Stargazey said...

Hi, Darwinstable! You had three comments, but the last two have somehow disappeared. Please feel free to re-post if you wish.

Some parts of the brain can use ketones, but it takes those tissues a while to switch over from using glucose. In the meantime, gluconeogenesis takes care of producing the required amount of glucose. But, in case it's not clear, the brain doesn't use the glucose to make dopamine. It simply uses the glucose as an energy source.

In case you're interested, another reason it's hard to give up carbs is that they can work as comfort food, via the neurotransmitter serotonin. When carbs are consumed, the resultant serotonergic and dopaminergic actions are different, but being aware of them can make it easier to devise ways to avoid the call of the carbs.

Kathy Hall said...

I've been reading all your back posts and I have a question. I started a low carb (under 40/day) diet about 6 weeks ago. I've been having stomach cramps. I don't have a gall bladder. I do take digestive enzymes. My husband is doing it with me and hasn't had any problems. Is this a common thing? I've tried taking baking soda since a lot of protein can be acidic but this helps only a little.

Stargazey said...

Hi, Kathy! I am not a physician and most definitely not a gastoenterologist, so these are just suggestions.

If you think it's a protein digestion issue, you might try using low-carb protein powder or low-carb protein shakes. They are easier to digest than meat. (I like the Body Fortress whey powder from Wal-Mart. It's cheap and is sweetened with Ace-K and Splenda.)

If it's a gall bladder issue, you will probably notice fatty stools (loose, foul-smelling, greasy, and they float). Steatorrhea is the medical term for it. You might want to eat smaller amounts of fat throughout the day rather than large amounts just at mealtime. You also may want to use fats like coconut oil, which is mostly medium chain fatty acids and is absorbed by a different mechanism than the longer-chain fatty acids found in liquid vegetable oils such as olive oil.

lynn said...

We know for a fact that long term use of cocaine leaves the user with depleted levels of dopamine even after being clean for years and decades. Could the same be true for those addicted to the sweet? Will they always have lower levels of dopamine than those not exposed to sweets?

Stargazey said...

I don't know, Lynn, but it wouldn't surprise me if prolonged exposure to the taste of sweet permanently downregulates dopamine signaling. I didn't see any references to it while I was preparing this blogpost, however.

In any case, given the many types of dopamine receptors and the many regulatory points in dopamine signaling, it doesn't seem obvious to me how a long-term decrease in dopamine signaling could be fixed pharmacologically.

When I did this blogpost, I was hoping that I might be able to help sweet-lovers make better food choices on a cognitive level. Until Jimmy Moore had his sweet-free challenges, I had no idea that simply the taste of sweet has an addictive component. Once a person is aware of this (assuming that there are no underlying psychological issues) perhaps he or she could then make conscious choices that would lessen the addictive power of sweet foods and beverages over their other food choices.